Method and device for converting carbon dioxide in flue gas into natural gas

ABSTRACT

A method for converting carbon dioxide in flue gas into natural gas using dump energy. The method includes: 1) transforming and rectifying a voltage of dump energy generated from a renewable energy plant, introducing the voltage-transformed and rectified dump energy into an electrolyte solution to electrolyze water therein to yield H 2  and O 2 ; 2) purifying industrial flue gas to separate CO 2  therein and purifying CO 2 ; 3) transporting H 2  generated and CO 2  to a synthesis equipment, allowing a methanation reaction between H 2  and CO 2  to happen to yield a high-temperature mixed gas with main ingredients of CH 4  and water vapor; 4) employing the high-temperature mixed gas to conduct indirect heat exchange with process water to yield superheated water vapor; 5) delivering the superheated water vapor to a turbine to generate electric energy, and returning the electric energy to step 1); and 6) condensing and drying the mixed gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International PatentApplication No. PCT/CN2013/074228 with an international filing date ofApr. 16, 2013, designating the United States, now pending, and furtherclaims priority benefits to Chinese Patent Application No.201210121972.7 filed Apr. 24, 2012. The contents of all of theaforementioned applications, including any intervening amendmentsthereto, are incorporated herein by reference. Inquiries from the publicto applicants or assignees concerning this document or the relatedapplications should be directed to: Matthias Scholl P.C., Attn.: Dr.Matthias Scholl Esq., 245 First Street, 18^(th) Floor, Cambridge, Mass.02142.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to technology for energy conversion of industrialflue gas using dump energy arising from renewable energy generation, andspecifically relates to a method and a device for converting carbondioxide in flue gas into natural gas by dump energy.

2. Description of the Related Art

There is an ongoing need to make full use of the dump energy arisingfrom renewable energy generation and further to effectively reduce thegreenhouse effect.

SUMMARY OF THE INVENTION

It is one objective of the invention to solve the defects of renewableenergy generation such as grid connection obstacles or being difficultto store short-time dump energy, and the problem that fossil energy hasenvironmental pollution caused by greenhouse gas, and provide a methodand a device for converting carbon dioxide in flue gas into natural gasby dump energy.

To achieve the above purpose, the key concept of the method designed inthe invention for converting carbon dioxide in flue gas into natural gasby dump energy is to first generate hydrogen through water electrolysisusing pump energy, and then synthesize natural gas easy for storage ortransportation through methanation reaction between hydrogen and carbondioxide trapped from industrial flue gas, which also facilitatesreasonable application of carbon dioxide discharged from industrial fluegas. The method comprises the following steps:

-   -   1) transforming and rectifying a voltage of dump energy        generated from a renewable energy plant, introducing the        voltage-transformed and rectified dump energy into an        electrolyte solution to electrolyze water therein to yield H₂        and O₂, and drying H₂;    -   2) purifying industrial flue gas to separate CO₂ therein and        purifying CO₂;    -   3) transporting H₂ generated from step 1) and CO₂ from step 2)        to a synthesis equipment comprising at least two fixed bed        reactors, allowing a methanation reaction between H₂ and CO₂ to        happen to yield a high-temperature mixed gas with main        ingredients of CH₄ and water vapor;    -   4) employing the high-temperature mixed gas generated from        step 3) to conduct indirect heat exchange with process water to        yield superheated water vapor;    -   5) delivering the superheated water vapor generated from step 4)        to a turbine to generate electric energy, and returning the        electric energy to step 1) for water electrolysis; and    -   6) condensing and drying the mixed gas in step 4) cooled through        the indirect heat exchange, until natural gas with CH₄ content        up to the standard is obtained.

The natural gas (SNG) can be sent to the existing natural gas pipenetwork through pressurized transport, or pressurized to liquefiednatural gas (LNG) for transport.

In a class of this embodiment, in step 1), the renewable energy isselected from solar energy, hydroenergy, wind energy, or a combinationthereof. These renewable energies are the most environment-friendly,cheapest and safest. The electrolyte solution is preferably potassiumhydroxide solution or other similar solutions with the density of1.2-1.4 kg/m³. Reaction temperature of the electrolyte solution iscontrolled at 90±2° C., and the reaction mechanism of water electrolysisis as follows: 2H₂O=2H₂↑+O₂↑. Compared with the pure water, theelectrolyte solution can significantly lower the electrolytic reactiontemperature, and save power consumption. After moisture removal andcooling of the resulting H₂ and O₂, H₂ may be used for the reaction inthe next step, while O₂ may be as a by-product for other usage.

In a class of this embodiment, various parameters of the fixed bedreactor at every stage in the above step 3) are as follows: inlettemperature: 250-300° C ., reaction pressure: 3-4 MPa, outlettemperature: 350-700° C . Methanation reaction mechanism of H₂ and CO₂is as follows: 4H₂+CO₂=CH₄+2H₂O+4160 kj/kg·CO₂. In specific operation,generally their mixture at a volume ratio of H₂:CO₂=4:1 is transferredto a fixed bed reactor for strong exothermic reaction in the presence ofa nickel-based catalyst or a similar catalyst, whilst releasing a lot ofheat, so that the temperature of the resulting mixed gas is greatlyimproved. At least two stage fixed bed reactors are provided to ensurecomplete reaction between H₂ and CO₂, and improve the utilizationefficiency of H₂.

In a class of this embodiment, in step 3), part of the high-temperaturemixed gas from the primary fixed bed reactor is bypassed for cooling,water removal, pressurization and heating, and is then mixed with freshH₂ and CO₂, so that the mixed gas is transported back to the primaryfixed bed reactor after the volume content of CO₂ therein is 6-8%. Inthis way, on the one hand, fresh H₂ and CO₂ can be preheated withreturning high-temperature gas to save energy consumption; on the otherhand, the reaction heat can be controlled through adjusting the volumecontent of CO₂, thereby controlling the highest outlet temperature ofthe fixed bed reactor, so that the catalyst is not deactivated atallowed temperature to ensure stable operation of the fixed bed reactor.

In a class of this embodiment, in a step 4), firstly, the process wateris heated to superheated water, which is then converted to yield watervapor, and finally the water vapor is converted to yield superheatedwater vapor. In this way, the process water is continuously, stably andreliably converted to yield superheated water vapor, so as to ensurethat the turbine always uninterruptedly generates power. The electricenergy generated thereby continues to be used for water electrolysis, sothat the high heat generated from the methanation reaction is fully usedto improve the conversion efficiency of the renewable energy.

In a class of this embodiment, in step 5), the steam exhaust generatedby the turbine after being driven for power generation is condensed towater, and then sent back to the process water line for recycling, so asto effectively improve the utilization efficiency of the process water,and save water resources.

In a class of this embodiment, in step 6), condensed water from themixed gas is transported back to the process water line for recycling,which can effectively improve the utilization efficiency of the processwater, and save water resources.

To achieve the above objectives, the invention also provides a devicefor converting carbon dioxide in flue gas into natural gas using dumpenergy. The device comprises a transformer and rectifier device, anelectrolytic cell, a turbine, a carbon dioxide heater, a primary fixedbed reactor, a secondary fixed bed reactor, a natural gas condenser, anda process water line. An outlet of the transformer and rectifier deviceis connected to a power interface of the electrolytic cell, a gas-liquidoutlet of a cathode of the electrolytic cell is connected to agas-liquid inlet of a hydrogen separator, a liquid outlet of thehydrogen separator is connected to a liquid reflux port of the cathodeof the electrolytic cell, a H₂ outlet of the hydrogen separator isconnected to an inlet of a hydrogen cooler, both the outlet of thehydrogen cooler and outlet of the carbon dioxide heater are connected toan inlet of the primary fixed bed reactor, an outlet of the primaryfixed bed reactor is connected to an inlet of the secondary fixed bedreactor successively through the superheater and mixed gas line of theprimary heat exchanger, and the outlet of the secondary fixed bedreactor is connected to the inlet of the natural gas condensersuccessively through the secondary heat exchanger and the mixed gas lineof the preheater. The process water line is connected to the aqueousmedium inlet of the preheater, the aqueous medium outlet of thepreheater is connected to the steam inlet of the superheater through asteam pocket, the steam outlet of the superheater is connected to thesteam inlet of the turbine, and the electric outlet of the turbine isconnected to the inlet of the transformer and rectifier device.

In a class of this embodiment, the mixed gas outlet of the primary heatexchanger is still provided with a bypass connected to the heat mediuminlet of a circulating heat exchanger, the heat medium outlet of thecirculating heat exchanger is connected to the inlet of a circulatingcompressor through a circulating cooler, the outlet of the circulatingcompressor is connected to the heated medium inlet of the circulatingheat exchanger, and the heated medium outlet of the circulating heatexchanger is connected to the inlet of the primary fixed bed reactor. Inthis way, a part of the high-temperature mixed gas generated from thereaction can reenter the primary fixed bed reactor by means ofself-circulation, so as to realize preheating the fresh H₂ and CO₂,reduce energy consumption and ensure continuous reaction.

In a class of this embodiment, an intermediate fixed bed reactor is alsoprovided between the above primary fixed bed reactor and secondary fixedbed reactor. The inlet of the intermediate fixed bed reactor isconnected to the mixed gas outlet of the primary heat exchanger, and theoutlet of the intermediate fixed bed reactor is connected to the inletof the secondary fixed bed reactor through an intermediate heatexchanger. In this way, in fact, three stage fixed bed reactors areprovided, so as to distribute the methanation reaction rate of H₂ andCO₂ stage by stage, until complete reaction of the raw materials. At thesame time, temperature of the fixed bed reactor can be reduced stage bystage, so as to obtain different quality of steam (temperature,pressure), and meet the needs of the turbine.

In a class of this embodiment, the steam exhaust outlet of the aboveturbine is connected to the process water line through the steam exhaustcondenser, which can save the water resources, and improve theutilization rate of process water.

In a class of this embodiment, the above process water line is stillconnected to the gas-liquid inlet of the hydrogen separator. In thisway, water can be transported to the electrolytic cell by the hydrogenseparator to supplement the water losses in the electrolytic reactionprocess and cool the heat generated from the water electrolysis process.

In a class of this embodiment, the condensed water outlet of the abovenatural gas condenser is connected to the aqueous medium inlet of thepreheater, so as to save the water resources, and improve theutilization rate of process water.

Advantages according to embodiments of the invention are summarized asfollows.

First, carbon dioxide trapped from industrial flue gas is converted toyield methane fuel (i.e., the main ingredient of natural gas) convenientfor storage and transport through methanation reaction with hydrogengenerated from water electrolysis by dump energy arising from therenewable energy generation, such as solar energy, hydroenergy, and windenergy etc. In this way, methane fuel is easily introduced into theexisting natural gas pipe network system, and may also be pressurized toliquefied natural gas (LNG) for transport by tank cars, therebyeffectively solving the above grid connection obstacle of dump energy orproblem of difficult storage of short-term dump energy.

Second, in the process of synthesizing methane using hydrogen and carbondioxide, huge amounts of carbon dioxide in flue gas is utilized, therebyachieving the goal of reducing carbon dioxide emission, solving theproblem of reducing huge amounts of carbon dioxide emission generated byfossil fuel, and bringing great economic benefits and social benefits.

Third, methanation reaction of hydrogen and carbon dioxide is a strongexothermic reaction, huge amounts of heat will be released in theprocess, the heat energy is used to produce high-temperature superheatedsteam to continue power generation, and then the electric energy is usedfor circulation of water electrolysis, thereby greatly improving theconversion efficiency of renewable energy.

Fourth, only methane and water vapor as the natural gas fuel are presentin the end product of methanation reaction of hydrogen and carbondioxide, and no other toxic by-products are available, which can notonly ensure the quality of the natural gas, but also reduce environmentpollution caused by greenhouse gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural diagram of a device for converting carbon dioxidein flue gas into natural gas by dump energy; and

FIG. 2 is a structural diagram of another device for converting carbondioxide in flue gas into natural gas by dump energy.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The method and device in the invention are further illustrated in detailin the light of the drawings and specific embodiments as follows:

Example 1

A device for converting carbon dioxide into natural gas by dump energy,as shown in FIG. 1, comprises a transformer and rectifier device 1, anelectrolytic cell 2, a turbine 4, a carbon dioxide heater 21, a primaryfixed bed reactor 13, a secondary fixed bed reactor 11, a natural gascondenser 8 and a process water line 3. The outlet of the transformerand rectifier device 1 is connected to the power interface of theelectrolytic cell 2. The gas-liquid outlet of the anode of theelectrolytic cell 2 is connected to the gas-liquid inlet of the oxygenseparator 20, liquid outlet of the oxygen separator 20 is connected tothe liquid reflux port of the anode of the electrolytic cell 2, O₂outlet of the oxygen separator 20 is connected to the inlet of theoxygen cooler 19, and outlet of the oxygen cooler 19 is connected to apressurized tank car or a filling device of O₂ (not shown in the figure)for other industrial use. The gas-liquid outlet of the cathode of theelectrolytic cell 2 is connected to the gas-liquid inlet of the hydrogenseparator 18, and the gas-liquid inlet of the hydrogen separator 18 isalso connected to the process water line 3 to supplement water losses.The liquid outlet of the hydrogen separator 18 is connected to theliquid reflux port of the cathode of the electrolytic cell 2, H₂ outletof the hydrogen separator 18 is connected to the inlet of the hydrogencooler 17, outlet of the hydrogen cooler 17 is connected to the outletof the carbon dioxide heater 21 and also connected to the inlet of theprimary fixed bed reactor 13, so as to transport fresh H₂ and CO₂ to theprimary fixed bed reactor 13.

Outlet of the primary fixed bed reactor 13 is connected to the inlet ofthe secondary fixed bed reactor 11 successively through a superheater 6and mixed gas line of a primary heat exchanger 7, the mixed gas outletof the primary heat exchanger 7 is still provided with a bypassconnected to the heat medium inlet of a circulating heat exchanger 16,the heat medium outlet of the circulating heat exchanger 16 is connectedto the inlet of a circulating compressor 14 through a circulating cooler15, the outlet of the circulating compressor 14 is connected to theheated medium inlet of the circulating heat exchanger 16, and the heatedmedium outlet of the circulating heat exchanger 16 is connected to theinlet of the primary fixed bed reactor 13.

Outlet of the secondary fixed bed reactor 11 is successively connectedto the inlet of a natural gas condenser 8 through a secondary heatexchanger 10 and mixed gas line of a preheater 9. The process water line3 is connected to the aqueous medium inlet of the preheater 9, aqueousmedium outlet of the preheater 9 is connected to the steam inlet of thesuperheater 6 through a steam pocket 12, the steam outlet of thesuperheater 6 is connected to the steam inlet of a turbine 4, and thesteam exhaust outlet of the turbine 4 is connected to the process waterline 3 through a steam exhaust condenser 5, and the electric outlet ofthe turbine 4 is connected to the inlet of the transformer and rectifierdevice 1 to provide electric energy for water electrolysis. In addition,the condensed water outlet of the natural gas condenser 8 may also beconnected to the aqueous medium inlet of the preheater 9 (not shown inthe figure) to send the condensed water back to the system forrecycling.

The process flow of the above device for converting carbon dioxide influe gas into natural gas by dump energy is as follows:

Dump energy arising from renewable energy generation, such as solarenergy, hydroenergy or wind energy etc., is converted to requiredcurrent through the transformer and rectifier device 1 to provideworking power supply for the electrolytic cell 2. Potassium hydroxidesolution with the density of 1.2-1.4 kg/m³ is used as the electrolytesolution within the electrolytic cell 2, and the reaction temperature iscontrolled at 90±2° C. Here, anode and cathode of the electrolytic cell2 respectively generate O₂ and H₂ carrying the electrolyte solution. Theelectrolyte solution is removed from O₂ generated therein with an oxygenseparator 20, and is transported back to the electrolytic cell 2 tofurther participate in the reaction. Afterwards, O₂ is cooled in anoxygen cooler 19 to 45° C. or so for water removal, and then deliveredto a pressurized tank car or a filling device for industrial use. Theelectrolyte solution is removed from H₂ generated therein with ahydrogen separator 18, and is transported back to the electrolytic cell2 to further participate in the reaction. Afterwards, H₂ is cooled in ahydrogen cooler 17 to 45° C . or so for water removal, and then entersthe reaction in the next step. Water losses in electrolysis isintroduced into the hydrogen separator 18 through the process water line3, is then supplemented to the electrolytic cell 2, and is also used tocool the heat generated in the water electrolysis process.

Meanwhile, CO₂ trapped from flue gas is purified, introduced into thecarbon dioxide heater 21, heated, and mixed with H₂ purified throughwater removal at the volume ratio of H₂:CO₂=4:1 to fresh gas, which istransported to the primary fixed bed reactor 13 for strong exothermicreaction (methanation). In order to control the reaction heat ofmethanation of H₂ and CO₂, certain amount of CH₄ may be added into theCO₂ heater 21 generally at the volume ratio of H₂:CO₂:CH₄=4:1:0.5.Addition of CH₄ can be stopped after the reaction is stable. The primaryfixed bed reactor 13 is kept at the inlet temperature of 250-300° C .,reaction pressure of 3-4 MPa, and outlet temperature of 600-700° C. Inthe presence of a nickel-based catalyst, most H₂ reacts with CO₂ togenerate high-temperature mixed gas of CH₄ and water vapor. Thehigh-temperature mixed gas is cooled to 250-300° C. successively throughthe superheater 6 and primary heat exchanger 7, and then divided intotwo parts. Where, a part of high-temperature mixed gas enters acirculating cooler 15 through the heat medium line of the circulatingheat exchanger 16, cooled to 30-40° C . after heat exchange, pressurizedto 3-4 MPa and heated to 180-200° C. with a circulating compressor 14,finally further heated to 250-300° C . through the heated medium line ofthe circulating heat exchanger 16, and mixed with fresh H₂ and CO₂ atsuch a ratio that the volume content of CO₂ in the mixed gas is 6-8%.The mixed gas is transported to the primary fixed bed reactor 13, andthe cycle is repeated. Preheating fresh H₂ and CO₂ in above circulationcan greatly reduce energy consumption and control the outlet temperatureof the primary fixed bed reactor 13. Another part of high-temperaturemixed gas is introduced into the secondary fixed bed reactor 11, whichis kept at the inlet temperature of 250-300° C ., reaction pressure of3-4 MPa, and outlet temperature of 350-500° C., so that the unreacted H₂and CO₂ therein continue to complete the strong exothermic reaction(methanation), until complete reaction of all raw materials.

The high-temperature mixed gas of CH₄ and water vapor from the secondaryfixed bed reactor 11 is cooled successively through a secondary heatexchanger 10 and a preheater 9, further cooled through a natural gascondenser 8, where gas CH₄ is cooled to 45-50° C., and flows out fromthe gas output of the natural gas condenser 8. CH₄ with the purity ofmore than 94% is pressurized to SNG/LNG (natural gas/liquefied naturalgas), and is transported through pipeline to the existing pipenetwork/tank car for storage and use; while the condensed water thereinflows out from the condensed water output of the natural gas condenser8, and is transported to the aqueous medium inlet of the preheater 9 forrecycling.

In the above strong exothermic reaction process of methanation, theprocess water is introduced into the preheater 9 through the processwater line 3, and is heated to superheated water through heat exchangetherein. Superheated water is transported to a steam pocket 12 throughpipeline to evaporate into water vapor therein. Water vapor istransported to the superheater 6 through pipeline to convert tosuperheated water vapor under given pressure by further heating. Thesuperheated vapor enters the turbine 4 through pipeline, the high-speedsuperheated water vapor drives the blades of the turbine 4 to rotate forpower generation, the generated energy returns to the transformer andrectifier device 1 for voltage transformation, rectification, andfurther use for water electrolysis, so as to make full use of the wasteheat in the strong exothermic reaction of methanation. The steam exhaustgenerated after the turbine is driven for power generation istransported to a steam exhaust condenser 5, and is condensed to water,which is transported back to the process water line 3 for recycling.

Example 2

Another device for converting carbon dioxide into natural gas by dumpenergy, as shown in FIG. 2, has the structure and process flow basicallythe same as that in Example 1, except that an intermediate fixed bedreactor 22 is provided between the primary fixed bed reactor 13 and thesecondary fixed bed reactor 11. The inlet of the intermediate fixed bedreactor 22 is connected to the mixed gas outlet of the primary heatexchanger 7, and the outlet of the intermediate fixed bed reactor 22 isconnected to the inlet of the secondary fixed bed reactor 11 through anintermediate heat exchanger 23. In this way, three stage fixed bedreactors are provided, so as to distribute the methanation reaction rateof H₂ and CO₂ in three stages, and ensure complete reaction of the rawmaterials. At the same time, inlet and outlet temperature of the threestage fixed bed reactors can be reduced successively, so as to obtaincorresponding quality of steam (temperature, pressure), and meet theneeds of the turbine 4.

The invention claimed is:
 1. A method for converting carbon dioxide influe gas into natural gas using dump energy, the method comprising: 1)transforming and rectifying a voltage of dump energy generated from arenewable energy plant, introducing the voltage-transformed andrectified dump energy into an electrolyte solution to electrolyze watertherein to yield H₂ and O₂, and drying H₂; 2) purifying industrial fluegas to separate CO₂ therein and purifying CO₂; 3) transporting H₂generated from step 1) and CO₂ from step 2) to a synthesis equipmentcomprising at least two fixed bed reactors, allowing a methanationreaction between H₂ and CO₂ to happen to yield a high-temperature mixedgas with main ingredients of CH₄ and water vapor; 4) employing thehigh-temperature mixed gas generated from step 3) to conduct indirectheat exchange with process water to yield superheated water vapor; 5)delivering the superheated water vapor generated from step 4) to aturbine to generate electric energy, and returning the electric energyto step 1) for water electrolysis; and 6) condensing and drying themixed gas in step 4) cooled through the indirect heat exchange, untilnatural gas with CH₄ content up to the standard is obtained.
 2. Themethod of claim 1, wherein the renewable energy is selected from solarenergy, hydroenergy, wind energy, or a combination thereof.
 3. Themethod of claim 1, wherein the electrolyte solution is a potassiumhydroxide solution with a density of 1.2-1.4 kg/m³, and a reactiontemperature of the electrolyte solution is controlled at 90±2° C.
 4. Themethod of claim 3, wherein in step 3), the fixed bed reactors have aninlet temperature of 250-300° C., a reaction pressure of 3-4 MPa, and anoutlet temperature of 350-700° C.
 5. The method of claim 3, wherein instep 3), part of the high-temperature mixed gas from a primary fixed bedreactor is bypassed for cooling, water removal, pressurization andheating, and is then mixed with fresh H₂ and CO₂, so that the mixed gasis transported back to the primary fixed bed reactor after a volumecontent of CO₂ therein is 6-8%.
 6. The method of claim 3, wherein instep 4), the process water is first heated to produce superheated water,which is then converted to yield water vapor, and finally the watervapor is converted to yield the superheated water vapor.
 7. The methodof claim 3, wherein in step 5), steam exhaust generated by a turbineafter being driven for power generation is condensed to water, and thensent back to a process water line for recycling.
 8. The method of claim3, wherein in step 6), condensed water from the mixed gas is transportedback to a process water line for recycling.